1. Field of the Invention
This invention relates generally to the field of mooring arrangements for vessels, particularly offshore vessels such as Floating Production and Offloading Vessels (FPSOs) or Floating Storage and Offloading vessels (FSOs) used in offshore hydrocarbon production. Still more particularly, the invention concerns active and passive damping arrangements for yoke/spring systems and yoke/pendulum systems which are spring-like to restore a vessel toward an equilibrium position with respect to a generally stationary body such as a tower or an anchored buoy.
2. Description of the Prior Art
There are numerous examples of yoke arrangements that couple a vessel to a body such as a tower or an anchored buoy or other generally stationary body. U.S. Pat. No. 4,290,158 shows a mooring yoke for a vessel which is coupled for rotation with a turntable on the top of the buoy. U.S. Pat. No. 4,309,955 shows a mooring yoke having two outer ends pivotably coupled to a vessel and having a counter weight on the yoke ends positioned outwardly beyond the coupling point of the vessel. U.S. Pat. No. 4,396,046 illustrates a yoke coupled between a mooring buoy and a vessel, where the yoke provides a base for a fluid conduit between a swivel on the buoy and fluid conduits on the vessel. U.S. Pat. No. 4,516,942 illustrates a yoke placed between a tower and a vessel, where ends of the two outer arms of the yoke are connected to the vessel by cables. Weights are positioned in the outer ends of the yoke arms, such that the yoke acts much like an undamped spring or a pendulum between the vessel and the tower. Dutch Patent 8602806 shows disconnectable yoke arms suspended from a tower. U.S. Pat. No. 4,530,302 shows a subsea yoke having its outer arms suspended by cables from the vessel. An enhanced pendulum effect is achieved by weight in the outer arms. The movement of the cables in the water increases damping of the spring effect of the weighted yoke arms. U.S. Pat. No. 4,665,856 shows a yoke coupled between a vessel and a tower. Weights are suspended from yoke arms near the tower. U.S. Pat. No. 4,694,771 also shows a yoke coupled between a vessel and a tower. Pendulum weights are provided on the yoke arms at their coupling to the tower. U.S. Pat. No. 4,568,295 shows a yoke positioned between an anchored buoy and a vessel, with the outer ends of the yoke arms suspended from the vessel and with a weight positioned on the yoke so that a pendulum arrangement is provided which acts like an undamped spring between the buoy and the vessel. U.S. Pat. No. 4,784,079 shows a tower-supported yoke suspended from a frame of a vessel with a pendulum weight provided at the end of the yoke arms. U.S. Pat. No. 4,917,038 shows a tower supported submerged yoke with quick-action couplings for disconnection. A weight at the end of the yoke suspended from cables or rods from the vessel causes the yoke to act like a pendulum or undamped spring, but the water acting on the suspension members and yoke damps the spring-like system more than if the yoke were entirely above water. U.S. Pat. No. 4,825,797 show other submerged yoke mooring systems.
The prior art described above provides mooring systems for vessel position control by relying on the deflection of a mechanical system to generate a spring-like restoring force, especially for tower/yoke systems. The damping of the tower-yoke-vessel systems arises primarily through friction of the vessel as it moves through the water in an oscillatory manner when environmental forces cause the vessel to move against its yoke.
Other mooring systems of course exist and all mooring systems can be generally categorized according to the type of restoring force produced as SALM or TLP systems, CALM systems or tower/yoke systems. In a SALM or TLP arrangement, the angular deflection of mooring legs result in inward mooring leg tension and an included angle to create a restoring force. In a CALM system, deflection of mooring legs increases mooring tension to produce a restoring force. In a tower/yoke system, deflection of pendular or spring systems results in a restoring force.
All three of the mooring categories described above have the following characteristics:
(1) The spring-like restoring forces are reactive and for that reason are not applied to the vessel until the vessel motion passes through the neutral or quiescent point;
(2) The damping force in the system is a small percentage of the spring-like restoring force; and
(3) As a consequence, momentum load on the mooring system is often a significant component of peak restoring loads, especially in body-yoke-vessel systems such as tower/yoke mooring systems.
In all of the stationary body-yoke-vessel arrangements described above, a mathematical model of a spring positioned between a stationary body and a movable body is appropriate with a small damping element placed in series with the spring. The stationary body is modeled as a fixed point. The mass of the movable body, i.e., the vessel, is very large. The momentum of the vessel causes it to move through the neutral point of the yoke and to move to the other side of it due to the system""s inherent lack of energy dissipation, and because the counter restoring force of the system cannot be generated until the vessel passes through the neutral or quiescent point to the other side. The natural damping force of the vessel moving through the water is not enough to prevent oscillatory motion of the vessel.
In other words, if a vessel is disturbed by wind, waves and current to a position away from its mooring neutral position, it obtains a potential energy with respect to the mooring devices which support it from a stationary point. The vessel is returned toward its neutral point with its potential energy converted into kinetic energy with the speed of the vessel increasing at the neutral point. This kinetic energy, if there is little or no damping in the mooring system, must be absorbed or converted into potential energy on the opposite side of the neutral point which requires greater vessel excursion or displacement from the neutral point than would be necessary in a mooring system with greater damping.
3. Identification of Objects of the Invention
A primary object of the invention is to provide an active xe2x80x9cforcing systemxe2x80x9d or active damping system by which excursions of a vessel past a neutral point of a yoke of a stationary body-yoke-vessel system are opposed by an active controlled restoring force. By applying such controlled force, displacement amplitudes of the vessel can be reduced or even eliminated, with the result that the overall size, weight and cost of the mooring system can be reduced.
Another object of the invention is to provide a passive damping system by which vessel oscillations past the neutral point are rapidly damped with the result that extreme displacement amplitudes of the vessel, that is amplitudes of the oscillation, are significantly reduced or even critically damped, with the result that a smaller system can be provided with reductions in size, weight and cost.
Another object of the invention is to provide a tower-yoke-vessel arrangement in which maximum displacement amplitudes of the vessel are small enough so that a product flow line from the tower to the vessel needs no supporting frame such as the yoke itself, but rather can be run from the top of the tower to the vessel.
Another object of the invention is to provide an arrangement by which forces are produced to control the motion of the vessel substantially independently of its displacement position magnitude from the mooring quiescent point.
Another object of the invention is to provide damping in a body-arm-vessel system through the use of pressure control devices coupled between the body and the arm or between the arm and the vessel.
The objects identified above, as well as other features and advantages are realized in several alternative embodiments of the invention described herein. An active damping system is provided in several embodiments where a signal is produced which is proportional to the displacement of the vessel from a neutral position of a body-yoke-vessel system. The signal controls the direction and magnitude of force of a cylinder linked to the yoke for applying a force to it in a direction opposite to that of its present motion.
A passive damping system is provided in other embodiments by which a damping hydraulic cylinder is applied in the yoke arms or arm so as to provide automatic passive damping force to a yoke with its ends connected directly to the vessel.
The invention includes arrangements with redundant cylinders by which both active and passive damping can be provided for a mooring arrangement. Components other than hydraulic cylinders can be used to achieve active and passive damping. Possible alternatives include brake shoes on linearly sliding structures or on rotating disks or drums all of which provide a damping force only. Cables from winches or drums are used in an active restoring force arrangement and/or a damping force. Electrical linear activators provide a restoring force and/or damping force. Elastomeric elements provide restoring and damping characteristics.
One embodiment of the invention includes a tower with a submerged yoke coupled to the tower and to the vessel. The tower includes a top section with a fluid swivel mounted on its top. Fluid conduits extend to the vessel from the tower-mounted swivel without the benefit of support from the submerged yoke.